The present invention relates to a new and improved method of constructing a plant containing installation for purifying a liquid as well as to a plant containing installation thus constructed.
The present invention also relates to a new and improved method of purifying a liquid by means of a plant containing installation of the aforementioned type.
In its more specific aspects, the present invention particularly relates to a new and improved method of constructing an installation for purifying liquids and which installation comprises a filter bed containing emerse helophyte plants and having an inlet for the liquid to be purified and an outlet for the purified liquid. A gravel bed is installed at the bottom of the filter bed and extends along part of the length of such filter bed. The bottom gravel bed is hydraulically connected to the inlet and defines a dam-up space.
Furthermore, the present invention specifically relates to a new and improved installation for purifying a liquid, such installation comprising a filter bed which contains emerse helophyte plants, an inlet, an outlet and a bottom gravel bed which extends along part of the length of the plant containing filter bed in the flow direction of the liquid to be purified. The bottom gravel bed is hydraulically connected to the inlet and defines a dam-up space.
In its further specific aspects, the present invention relates to a new and improved method of purifying a liquid by passing the same from an inlet through a filter bed containing emerse helophyte plants to an outlet and through a bottom gravel bed which extends along part of the length of the filter bed in the flow direction of the liquid to be purified. The bottom gravel bed is hydraulically connected to the inlet and defines a dam-up space.
Such methods and installations have been described in addition to earlier publications in Published European Patent Application No. 0 243 678 which is cognate with U.S. Pat. No. 4,855,040, granted Aug. 8, 1989, and U.S. Pat. No. 4,904,386, granted Feb. 27, 1990. In the known method and the known installation, the bottom gravel bed is penetrated by lengthwise extending discharge pipes which are provided with adjusting means for adjusting the throughflow on the outlet side. In this manner the bottom gravel bed inclusive of the discharge pipes defines a by-pass flow path bypassing the soil matrix of the plant containing filter bed; this by-pass flow path takes up part of the inflow and enables the throughflow through the plant containing filter bed to be externally adjusted on the outlet side, i.e. externally of the plant containing filter bed.
Among modern methods of waste water treatment the so-called plant waste water treatment installations play an increasingly important role. Therein the waste water purification is effected by means of a passage through a plant containing soil body capable of purifying even uncommon waste waters due to its density of microbial activity and its variety of excessive purification powers. Particular attention is paid to the extremely low utilization of external energy as compared to more conventional processes like the activated sludge process or aerated basin waste water treatment methods.
The designation "plant waste water treatment installation" does not represent a particularly well selected term in view of the fact that the actually effective section therein is constituted by the through-flown soil body which experiences a known physical, chemical and biological activation caused by the plants and which does not require a more detailled description in connection with the subject matter of the instant invention.
The so-called "plant waste water treatment installations" have become additionally attractive because it has been found meanwhile that the initially overestimated specific surface area requirement is not at all excessive but is in the range of 2 m.sup.2 to 10 m.sup.2 per population equivalent.
The surface area required for such installation is computed on the basis of the kinetic values of the degradation reactions of the organic load as expressed in terms of BOD.sub.5, via a relationship which represents the required surface area as a function of the waste water volume and the extent of its contamination as well as the intended degree of purification: EQU F.sub.x =5.2.multidot.Qd.multidot.ln(C.sub.0 /C.sub.t) (I)
Therein
F.sub.x is the required surface area in m.sup.2, PA1 Qd is the daily volume of water in m.sup.3, PA1 C.sub.0 is the inlet concentration of BOD.sub.5 in mg/l, and PA1 C.sub.t is the outlet concentration of BOD.sub.5 in mg/l, i.e. the extent of purification to be achieved. PA1 .phi. is the infiltration cross-sectional area, i.e. the input surface area (flow frame) for the waste water in m.sup.2, PA1 Q is the inflow of waste water or the throughflow thereof in m.sup.3 /sec, and PA1 v is the advance rate or linear flow rate of the flowing wave in the through-flown soil body in m/sec, i.e. the filtration rate. PA1 v is the filtration rate in m/sec, PA1 k.sub.f is the permeability coefficient of the soil body in m/sec, and PA1 .DELTA.h/.DELTA.s is the hydraulic gradient. PA1 7.6 m in the flow direction (flow distance) and PA1 1,853.00 m of the infiltration width or breadth. PA1 18.2 m flow distance and PA1 772.00 m of the infiltration width or breadth.
The module 5.2 is a specific quantity which results from the kinetic reaction constants of the BOD.sub.5 degradation.
The thus determined surface area, however, can not have any desired configuration in horizontally through-flown plant containing filter beds which are the predominantly concerned filter beds, because a predetermined infiltration or throughflow cross-sectional area is, of course, required for the passage or transport of the water volume Qd through the soil body. This infiltration or throughflow cross-sectional area is defined by the rate of flow or filtration v permitted by the soil body, as will be self-evident.
Generally, the transport occurring in the horizontally through-flown filter bed is described by the continuity equation EQU .phi.=Q/v (II)
Therein
According to Darcy, the flow rate or filtration rate in a soil body can be represented by the following equation: EQU v=k.sub.f .multidot..DELTA.h/.DELTA.s (III)
Therein
Plant waste water treatment installations, particularly the so-called "rootspace beds" which are most consequently based on the productivity of root-permeated top soil under a growth of emerse helophytes, generally are dimensioned for a predetermined depth, mostly 0.6 m, of active space. At the bottom, they are hermetically sealed from the subsoil or the ground water body.
In the thus defined active space or soil matrix there are achieved high permeability coefficients k.sub.f due to the particular selection of the soil but primarily due to the secondary structuring activity of the subterraneous plant organs (roots and rhizoms). The thus obtained permeability coefficients have a magnitude which otherwise is known only in connection with coarser primary granulations.
Thus it could be shown that, subsequent to root permeation for several years of an initial soil having a permeability coefficient of k.sub.f =10.sup.-7 m/sec, there can be formed a plant containing filter bed having a k.sub.f value of 5.multidot.10.sup.-3 m/sec, a k.sub.f value which otherwise is found only in coarse sands. Nevertheless, such high permeability is rather an exception; it is, however, ensured that k.sub.f values of 5.multidot.10.sup.-4 m/sec can be obtained.
It is herein that there exist system-based problems with regard to the dimensioning and the use of such installations as will be shown hereinbelow.
Computation of a root space installation, which is designed for a population equivalent of 5,000, is based on the usual parameters. Thus there is expected a daily waste water arrival Qd of 750 m.sup.3. Assuming an infiltrated waste water proportion of 50 l per population equivalent and day, the inflow into the purification installation will be 1,000 m.sup.3 /d. When the computation is based on a time period of 10 hrs., an inflow rate of 2.78.multidot.10.sup.-2 m.sup.3 /sec will result.
In accordance with common standard data after mechanical prepurification, a BOD load of 45 g per population equivalent and day is assumed for the waste water. The total infeed concentration thus is 225 mg/l BOD.sub.5.
The purification goal is intended to be 15 mg/l BOD.sub.5.
The surface area required for the plant containing filter bed, then, is EQU Fx=5.2.multidot.1000 ln (225/15)
in accordance with equation (I) which results in EQU Fx=14,082 m.sup.2
under the indicated conditions. There is thus obtained a specific surface area requirement of 2.82 m.sup.2 per population equivalent. This can be readily realized technically and also has been realized frequently.
However, realization of the required infiltration or flow cross-sectional area presents a significant problem as will be shown subsequently. According to equation (II) the flow cross-sectional area is ##EQU1##
Assuming that a hydraulic gradient .DELTA.h/.DELTA.s of 5% can be realized which may be accomplished by means of a corresponding bottom slope in the direction of flow, there will result ##EQU2##
In the event of a depth of the active region or profile depth of 0.6 m, this would translate into an installation width or breadth of 1,853 m (?).
The total surface area of the installation amounts to 14,082 m.sup.2. The configuration of the installation thus would be defined by the following dimensions:
Such a hydraulically caused configuration of the surface area can not be managed with regard to the distribution of the liquid to be purified and, considering the passage length of only 7.6 m, can not result in a reliable throughflow pattern and reliable purification.
The system-based problems of such purification processes thus have been demonstrated by the aforediscussed example.
In view of the extraordinary other advantages offered by these purification processes there has been no lack of experiments and proposals for overcoming the heretofore mentioned difficulties.
Firstly, one would be justified to base the operation on a total installation inflow which is uniformly distributed through 24 hrs. This may be accomplished technically, for example, by employing an equalizing basin. Also, the large buffer capacity of the plant containing filter bed as such may be taken into consideration. There is thus obtained an inflow of 1.16.multidot.10.sup.-2 m.sup.3 /sec with the result of
There is not much gained in principle by these measures because also this configuration of the surface area will result in hardly surmountable distribution problems and a passage length of 18.2 m hardly will produce a homogeneous throughflow pattern.
Only marginally are here to be mentioned the problems connected with economically positioning such surface area in the terrain.
Practical experiments of significantly increasing the bottom slope, for example, to about 10%, result in different hydraulic difficulties, for instance, in discharge of water through the surface of the soil body prior to termination of the passage. Furthermore, also in this case, there can not be produced a surface area of a configuration which would be technically realizable and which would be desirable in terms of waste water technology.
In a few cases recourse was taken to subdividing the surface area into a number of partial surface areas but also this measure soon reaches its limits. Above all, the technical expense increases to such significance that reasonable construction costs can no longer be realised. Each part-bed must have input and output units provided with respective adjusting means and the same or proportional waste water volume must be allocated to the different part-beds. No useful technical solution has hitherto been developed for solving this problem.
A last known possibility of solving the hydraulic problems while maintaining the passage through the soil resides in selecting a vertical infiltration mode. This is comparatively frequently put into use although this process variant has its particular difficulties and also its imponderabilities. Firstly, it is difficult to achieve uniform distribution of the waste water across larger surface areas. In most cases distribution by spraying is out of consideration due to the nuisance connected therewith. Likewise, a free, open waste water surface on the soil body is generally not considered acceptable. Above all, however, the k.sub.f values are differently developed and any counterregulation is impossible. Also, a passage through a maximum of 1 m of active root space is rarely sufficient for reliable purification through contact with the active structures present in the soil body.
Numerous project engineers, therefore, have turned away from employing the highly complex and highly active soil matrix of plant containing filter beds and utilize instead throughflown gravel beds and coarse sand beds for waste water purification, evidently inclusive of all the losses in purification power and biochemical versatility, which are properties characterizing a soil and also desirable in view of the xenobiotica always present even in domestic waste water.